At present, as a flexible copper-clad laminated sheet for use in mobile electronic apparatuses, a material obtained by bonding copper foil to the surface of a polyimide film or a polyester film with adhesive is a mainstream. In these years, mobile electronic apparatuses are made more compact, lightweight and highly functional, whereby flexible circuit boards having very fine circuit patterns and being excellent in high-frequency characteristics are demanded. For example, as the line space of very fine circuit patterns, a shape measuring 25 μm or less, for example, is demanded, and the demand for higher accuracy is also increasing to meet the need for bare chip mounting. In a conventional film obtained by bonding copper foil to a plastic film, the need for high-density thin-line design is satisfied by improving the heat resistance of adhesive and by making the copper foil thinner; however, the need for lightweight and highly functional design has not yet been satisfied sufficiently. In addition, the conventional film has problems to be solved, such as reduction in dimensional accuracy and reduction in electrical characteristics owing to the use of adhesive.
Furthermore, in a method for forming very fine circuit patterns having a thickness of 10 μm or less, the handling in a step of bonding the very fine circuit patterns to a film substrate is particularly difficult, thereby causing problems, such as the generation of creases and the adhesion of impurities.
As methods for directly forming a film of copper on a plastic film substrate to solve the above-mentioned problems in the conventional film, the vacuum evaporation method, sputtering method, ion plating method and copper plating method have been examined. These film forming methods have an effect of eliminating the above-mentioned defects owing to adhesive since there is no adhesive between the film substrate and the copper thin film and also has an effect of obtaining a printed circuit board excellent in electrical characteristics, such as electrical insulation and dielectric constant.
However, these film forming methods have problems; long time is required to form a necessary thickness of 5 to 10 μm, and the adhesion strength between the plastic film substrate and the copper thin film is insufficient.
Furthermore, a method for forming a copper thin film on a plastic film substrate by using the evaporation plating method has been examined. However, this evaporation plating method requires acid processing during plating processing, thereby causing a problem of delaminating an evaporated film during this acid processing.
A method for producing a conventional flexible printed circuit board will be described below briefly.
FIG. 15 is a cross-sectional view showing the configuration of a printed circuit board disclosed in the Official Gazette of Examined Japanese Patent Publication No. Sho 57-33718. As shown in FIG. 15, the Official Gazette of Examined Japanese Patent Publication No. Sho 57-33718 discloses a printed circuit board produced by evaporating a metal, such as nickel, cobalt or palladium, as an intermediate layer 12 on a plastic film substrate 11, and after the evaporation processing, by further evaporating a copper thin film 13. The peeling strength of the flexible printed circuit board has been improved by carrying out production in this way. However, in the case when the printed circuit board is left stationary in an environment of supersaturated steam of 120° C. at 2 atm after being subjected to electrolytic copper plating, there is a problem of being unable to maintain its desired peeling strength. In addition, in this production method, a step of forming a metal film serving as the intermediate layer 12 is necessary; for this purpose, etching processing and dewatering processing are required to be carried out, thereby having a problem of increasing cost.
The Official Gazette of Unexamined Japanese Patent Publication No. Hei 8-330728 proposes a method wherein chromium, an chromium alloy or the like is evaporated on a polyimide film containing 0.02 to 1 wt % of tin and then copper is evaporated thereon. However, this method has problems; insulation performance reduces since tin is present in the polyimide film, and etching efficiency lowers since an anchor evaporation layer for copper evaporation is required. Furthermore, this method also has problems to be solved; cost for forming a film of a chromium-based alloy and cost for dewatering processing are required, and environmental loads are large.
The specification of Japanese Patent Publication No. 2982851 (the Official Gazette of Unexamined Japanese Patent Publication No. Hei 6-228738) discloses a production method wherein copper sputtering is carried out in an atmosphere containing nitrogen at a volumetric ratio of approximately 0.09% to approximately 50% in order to improve the adhesion strength between a plastic film substrate and a copper thin film. However, the kinetic energy of the copper atom in sputtering is 5 eV at most (50 eV at a maximum), and the peeling strength has fluctuations depending on the surface state of the plastic film, thereby causing a problem of being unable to obtain a peeling strength of 1 kg/cm, the target of the product.
The Official Gazette of Unexamined Japanese Patent Publication No. Hei 5-251511 proposes a production method wherein after a rough face (microscopic irregularity) is formed by carrying out dry etching on the surface of a polyimide film using a mixture gas, such as oxygen, the rough face is subjected to discharge plasma processing using a gas containing nitrogen so as to generate functional groups having a strong bonding action with copper on the surface of the polyimide film. After the functional groups having the strong bonding action with copper are generated on the surface of the polyimide film, copper is evaporated on the surface to form a copper thin film. In this production method, it is certain that the peeling strength tends to be improved; however, the etching characteristics and the effect of the plasma processing varies depending on the surface state of the polyimide film. In other words, depending on the surface state of the polyimide film, the degree of the irregularity formed by the etching varies and the degree of the generation of the functional groups differs. Hence, in the polyimide flexible printed circuit board produced by the production method disclosed in the Official Gazette of Unexamined Japanese Patent Publication No. Hei 5-251511, the peeling strength of the copper thin film has fluctuations and does not satisfy a desired strength in some cases. In addition, in the disclosed production method, the plasma processing is carried out by using a reactive ion etching apparatus; however, in this kind of production method, the kinetic energy of ionized nitrogen plasma is 5 eV to 10 eV at most, whereby the generation of the functional groups is insufficient. Furthermore, since the kinetic energy in this production method is low, the generation of functional groups is greatly depending on the surface state of the plastic film, thereby causing a problem of being unable to securely generate functional groups on the plastic film.
In the thin-film production apparatus described in the specification of Japanese Patent Publication No. 890408 (the Official Gazette of Unexamined Japanese Patent Publication No. Sho 49-61031), a thin-film forming apparatus is used wherein high-frequency power is applied via a capacitor to a conductive substrate holder in which a substrate is installed and a substantially negative self-induced voltage is generated, whereby the peeling strength is improved. However, the specific conditions of forming a metal thin film on a plastic film, the relationship in peeling strength between the plastic film and the metal thin film and the relation between a film structure having high peeling strength and film-forming conditions are not disclosed in the Patent Publication No. 890408 at all.
The Official Gazette of Unexamined Japanese Patent Publication No. Hei 11-117060 discloses forming a modified polyimide layer by processing the surface layer of the polyimide film by using physical means, such as plasma or corona discharge, or by using chemical means, such as chemicals. This modified polyimide layer has a molecular structure comprising a carboxyl group generated when an imide ring is opened and at least one hydroxyl group attached to a benzene ring bound to the nitrogen of the imide ring and/or the nitrogen of the second amide generated when the imide ring is opened. In the technology disclosed in the Official Gazette of Unexamined Japanese Patent Publication No. Hei 11-117060, the modified polyimide layer formed is coated with a metal, such as copper, by a dry plating method, such as vacuum evaporation, sputtering and ion plating. However, the oxygen of the hydroxyl group changes the oxidation of copper from Cu2O to CuO and degrades adhesion performance, thereby causing fluctuations in the peeling strength of the copper foil film.
The Official Gazette of Unexamined Japanese Patent Publication No. Sho 64-8264 discloses a film-forming method wherein metal grains, such as copper, having passed through the plasma of a mixture gas containing argon containing 0.01 to 10% of oxygen are attached to a plastic film subjected to plasma processing at low temperatures using oxygen gas. However, just as in the case of the technology disclosed in the above-mentioned Official Gazette of Unexamined Japanese Patent Publication No. Hei 11-117060, the oxygen of the hydroxyl group formed in the surface layer of the plastic film changes the oxidation of copper from Cu2O to CuO and degrades adhesion performance, thereby causing fluctuations in the peeling strength of the copper foil film.
The Official Gazette of Unexamined Japanese Patent Publication No. Hei 11-92917 discloses reforming the surface of a polyimide film by plasma processing and forming a metal thin film, such as chromium, nickel or titanium on the reformed surface. In addition, it is informed that a copper film is taught by vacuum evaporation on the above-mentioned metal thin film and that electric copper plating is carried out further thereon. In the plasma reforming disclosed in the Official Gazette of Unexamined Japanese Patent Publication No. Hei 11-92917, the reformation is carried out so that the ratio (O/C) of the oxygen and carbon of the polyimide film is between 0.26 and 0.45. In this film-producing method, just as in the case of the technologies disclosed in the above-mentioned Official Gazette of Unexamined Japanese Patent Publication No. Hei 11-117060 and Official Gazette of Unexamined Japanese Patent Publication No. Sho 64-8264, the oxygen captured in the film changes the oxidation of copper from Cu2O to CuO and degrades adhesion performance, thereby causing fluctuations in the peeling strength of the copper foil film.
The specification of U.S. Pat. No. 5,178,962 discloses a technology wherein the surface of a polyimide film is subjected to plasma processing by using an apparatus for generating electron cyclotron resonance plasma (Electron Cyclotron Resonance plasma generator: ECR plasma generator) to generate functional groups containing nitrogen, belonging to the carbonyl group, carboxyl group, amide group, amino group, imino group, imide group and cyano group, and to generate metal, while the degradation of the polyimide film is suppressed. In the technology disclosed in the specification of U.S. Pat. No. 5,178,962, the nitrogen concentration of polyimide increases after surface reformation using nitrogen or ammonia gas. However, since the kinetic energy is low, 50 eV or less, the oxygen concentration cannot be reduced by the ECR plasma processing. The oxygen of the surface layer of the polyimide film is a factor of degrading the reliability of a plastic film substrate on which copper is directly deposited. Furthermore, in the ECR plasma processing, the structure of the apparatus is limited so as to satisfy a cyclotron frequency of 2.45 GHz, and this ECR plasma processing apparatus and an evaporation apparatus are required to be arranged on one production line. Hence, problems are caused; the production apparatus becomes a large-scale apparatus, the surface reformation processing and the evaporation processing are long, complicated and high in cost.
The Official Gazette of Unexamined Japanese Patent Publication No. Hei 5-287500 discloses a circuit board production method wherein metal vapor is evaporated on a film, and the film is irradiated with nitrogen gas ions accelerated from an ion source simultaneously or alternately with the evaporation. According to this circuit board production method, the mixture layer of a metal element and nitrogen is formed at the interface between the metal layer and the polyimide, thereby improving adhesion performance. However, according to the circuit board production method disclosed in the Official Gazette of Unexamined Japanese Patent Publication No. Hei 5-287500, the acceleration voltage of the nitrogen ions is a high value of 5 KeV; hence, there is a fear of degradation of the polyimide owing to high-energy ions.
The Official Gazette of Unexamined Japanese Patent Publication No. 2001-151916 discloses a production method wherein a polyimide film containing aluminum oxide or silicon dioxide is subjected to plasma processing and a copper thin film is formed by the ion plating method using pressure gradient discharge. In the production method disclosed in the Official Gazette of Unexamined Japanese Patent Publication No. 2001-151916, when the copper thin film is formed by the ion plating method, the copper thin film is determined that the X-ray relative intensity ratio between crystal lattice plane indices (200)/(111) is 0.37 to 0.4. However, pattern defects occur in the case when aluminum oxide or silicon dioxide appears on very fine circuit patterns. In addition, crystals with crystal lattice plane indices (200) and (111) are mixed, numerous grain boundaries are generated between the two crystals, thereby causing a problem of degradation in chemical resistance. Furthermore, since the etching speed varies depending on crystal and grain boundary, there is a problem of varying the dimensional accuracy of the very fine circuit patterns.
An object of the present invention is to solve the problems in the above-mentioned various conventional technologies and to provide a technology for bonding a copper thin film made of copper or an alloy containing primarily copper to a plastic film firmly and directly.
In particular, an object of the present invention is to provide a flexible printed circuit board, at low cost, in which a copper thin film made of copper or an alloy containing primarily copper is directly bonded with adhesive to at least one side of a plastic film, which has very high adhesion performance and on which very fine circuit patterns can be formed by etching, wherein the interface structure between the plastic film and the copper thin film and the crystalline structure of a copper thin film to be grown subsequently are controlled so as to have desired structures by controlling the state of the plastic film substrate on which a film is formed and by optimizing the film-forming conditions of the copper thin film.